Various types of carbon dioxide generators are known in the prior art. However, what is needed is a short chain hydrocarbon combustor and carbon dioxide generator that is installable into open communication with an existing ventilation system operative to vent heat from an indoor grow space, said short chain hydrocarbon combustor and carbon dioxide generator enabling air-to-air heat exchange at each of a first heat exchanger and a second heat exchanger, wherein at least 95% of heat of combustion is transferred from exhaust gas into airflow routed through the ventilation system exterior to the grow space and water vapor is condensable and collectable from said exhaust gas whereby cooled carbon dioxide is producible into the grow space upon demand by action of a demand relay disposed in operational communication with a gas valve and an igniter, said carbon dioxide producible at desired quantities without appreciably effecting the thermal load and relative humidity occurent in the operative grow space.